Rear paraboloid feed for round guide



Feb. 22, 1955 J. s. FOSTER 2,702,858

REAR PARABOLOID FEED FOR ROUND GUIDE Filed Sept. 19, 1945 FIG. I

INVENTOR Joy/v Sum/er 2 0x752 ATTORNEY United States Patent REAR PARABOLOID FEED FOR ROUND GUIDE John Stuart Foster, Montreal, Quebec, Canada, assignor, by inesne assignments, to the United States of America as represented by the Secretary of the Navy Application September 19, 1945, Serial No. 617,364

Claims. (Cl. 250-33.65)

This invention relates in general to directional microwave antennas and in particular to apparatus for illuminating a paraboloidal reflector with energy from a cylindrical or so-called circular wave guide.

Since in many applications it is desirable that the paraboloidal antenna feed be symmetrical to the axis of the reflector the guide commonly projects through the reflector and along its axis to the neighborhood of its focus where there is located some sort of feed or iiluminating device to cause the energy from the guide to travel back toward the paraboloidal reflector. It is also desirable for certain applications to obtain a radiation beam pattern from such a system having a narrow main beam and with minimized minor lobes.

A well known method of exciting a paraboloid antenna is by use of a dipole element located at or near the focus. Other methods include the use of wave guides introduced substantially through the back of the reflector with reflecting means positioned in front of the mouth of the guide near the focus of the paraboloid to reflect toward the paraboloid the energy emitted from the guide.

Difficulties were experienced under the prior art in that a circular wave guide is relatively large in cross section compared with an equivalent rectangular guide, and consequently when a circular guide was employed in the manner above described the guide itself presented a sizable and serious obstacle to the illumination of the main reflector from the small reflecting or exciting elements; and side or minor lobes were produced of high ratio to the size of the main lobe.

It is therefore a principal object of the present invention to provide means for illuminating a paraboloid reflector from energy in a circular wave guide so that highly directive radiation beams with relatively small secondary lobes may be produced.

A correlative object is to provide a paraboloid for receiving microwaves having a pattern which is highly directive and relatively free of gain in directions other than that of the main pattern, which system is adapted for conveyance of such received energy within circular wave guides.

A further object is to provide, in a circular wave guide, means for uniformly illuminating a paraboloidal reflector.

It is a further object to provide a wave guide of tapering construction so as to reduce the obstruction between the auxiliary reflecting element and the principal paraboloidal reflector.

Another object is to provide a proper impedance matching between the wave guide feed and the tapered guide and auxiliary reflecting means.

Other objects and advantages will be found in the following detailed description together with the drawings, the figures of which are for illustrative purposes and are not to be deemed limitational.

Fig. 1 is a sectional view of the preferred embodiment and Fig. 2 is a partial sectional view of an embodiment similar to that of Fig. l but having a different auxiliary reflector.

Referring now to Fig. 1, there is shown, in part, a paraboloidal reflector 1 having a cylindrical wave guide 3 extending through it from the rear usually at the vertex and substantially along the axis toward the focus. Beginning some distance inside the focus at point indicated as 5 the guide 3 is tapered to a point near the 2,702,858 Patented Feb. 22, 1955 end marked 7 and then maintains the diameter of the smaller end of the taper for a short distance to the end of the guide at 9. In order to keep the optical diameter constant an insert 11 of polystyrene or some similar dielectric material may be used having a conical portion removed therefrom with apex 13 of said portion pointing toward the forward end 9 of the guide.

The energy issuing from the end 9 of the guide 3 is reflected back to the main paraboloidal reflector 1 by a circular metallic resonant ring 15 positioned near the focus by a polystyrene diskor washer 16 supported in turnlliy a member 17 mounted into the polystyrene insert Beyond the focus a short distance a second circular resonant ring 19 is supported by a similar polystyrene disk 21 also mounted on the member 17. This ring 19 acts parasitically to direct the radiation back into the paraboloidal reflector in a manner favorable to achievement of high gain and low side lobe intensity.

Because of the tapering of the end of the circular guide less obstruction is presented to radiation from the rings 15 and 19 toward the principal reflector 1. It is of course possible to use resonant rings as the primary antenna elements, fed from an untapered circular guide, but less usefully. Impedance along the guide is maintained constant by the tapered dielectric insert 11 in the cases illustrated.

The construction of an alternative embodiment is illustrated by Fig. 2 which shows in part the same sort of tapered circular wave guide 31 as was described with reference to Fig. 1, having the same sort of tapered dielectric insert 32 and a threaded axially positioned supporting member 33 mounting a polystyrene disk 34 on which is mounted a metallic resonant ring 35 to serve as the primary antenna element fed by energy from the guide 32. However, in place of the second resonant ring of Fig. 1 there is placed forward of the radiating ring a metallic disk 36 to reflect forward radiation from the ring 35 back toward a principal paraboloidal reflec- {og (not shown) for the purpose of narrowing the main The principle of tapering the wave guide at the outlet end and using an inserted wedge of dielectric to maintain the electrical dimensions and characteristics unaltered may be applied equally well to rectangular wave guides as to the circular.

Also the resonant ring may be found useful as a primary element in apparatus not using the tapered guide.

What is claimed is:

l. A micro-wave directional antenna system comprising a paraboloidal reflector, an open ended circular wave guide extending axially through said reflector and being tapered toward the focus of said paraboloidal reflector to a smaller cross section at its open end, a tapered dielectric member positioned within the tapered portion of said guide, said member being constructed in such manner that as the wave guide cross section is reduced the amount of dielectric material within the guide is increased for maintaining a constant electrical size of the guide as the physical dimensions decrease, a metallic ring axially disposed from the end of said tapered guide and near saidfocus, means for supporting said ring, a second metallic ring axially disposed from said first ring farther away from the open end of the guide, and means for mounting said rlng.

2. A micro-wave directional antenna system comprising a paraboloidal reflector, a wave guide having an open end and extending axially through said reflector and being tapered toward the focus to a smaller cross section at the open end, a tapered dielectric member positioned within the tapered portion of said guide, a metallic ring axially disposed from the end of said guide and substantially at the focus of said paraboloidal reflector, means for positioning and supporting said ring, a second metallic ring axially disposed from said first ling farther from the end of said guide, and means for positioning and supporting said second ring.

3. A directional microwave antenna system comprising a paraboloidal reflector, a wave guide having an open end extending through said reflector substantially axially, said wave guide being tapered near its open end, a plug composed of dielectric material positioned within said tapered portion of said guide, said plug being hollowed to provide a conical space therein Whose axis coincides with the axis of the tapered guide and whose vertex is toward the smaller end of said plug, a disk of dielectric material, a supporting member attached to the end of said dielectric plug for mounting said disk perpendicular to the axis substantially at the focus, a metallic ring mounted circumferentially on said disk, a disk of dielectric material mounted on said supporting member perpendicular to said axis and a fractional wave-length away from said focus and reflector, and a metallic ring mounted circumferentially on said last named disk.

4. A directional microwave antenna system compris' ing, a paraboloidal reflector, a wave guide extending substantially axially through said reflector, said Wa e guide comprising a first cylindrical section filled with a first dielectric material, a second coaxial cylindrical section having a smaller cross section than said first section and filled with a second dielectric material, a tapered section of circular cross section connecting said first and second sections, a tapered member comprising first and second sections formed respectively of first and second dielectric materials and positioned within said tapered section of wave guide, said second section of wave guide having an open end at the end opposite said tapered section of wave guide, a first disk of dielectric material, a supporting member mounted in said second dielectric material at said open end of said second section of wave guide, said first dielectric disk being mounted to said supporting member perpendicular to the axis of said reflector substantially at the focus of said reflector, a first metallic ring mounted circumferentially on said disk, a second disk of dielectric material mounted on said supporting member perpendicular to said axis and a fractional wave length away from said focus in a direction away from said reflector, and a second metallic ring mounted circumferentially on said second dielectric disk.

5. Apparatus as in claim 4 wherein said first section of said first dielectric material is in the shape of a cone, the base of said cone being coincident with the plane of the junction of said first wave guide section and said tapered wave guide section, the apex of said cone being coincident with the center of said second wave guide section at the junction of said second wave guide section and said tapered wave guide section, and wherein second section of said second dielectric material occupies the space in said tapered wave guide section included between the wall of said tapered Wave guide section and said cone.

References Cited in the file of this patent UNITED STATES PATENTS 2,054,895 Dallenbach Sept. 22, 1936 2,206,923 Southworth July 9, 1940 2,406,945 Fell Sept. 3, 1946 2,411,534 Fox Nov. 26, 1946 2,422,184 Cutler June 17, 1947 2,427,098 Keizer Sept. 9, 1947 2,429,640 Mieher Oct. 28, 1947 2,497,706 Wetherill Feb. 14, 1950 2,509,196 Cork et al. May 23, 1950 2,531,455 Barrow et al. Nov. 28, 1950 

